The invention relates to electrical neurostimulation, wherein electric currents are passed to the brain, the spinal cord, an organ, or peripheral nerves 1-3!. Such stimulation has been used with various degrees of success for anesthesia, induction of relaxation and sleep, as well for the treatment of pain, intractable epilepsy, behavioral disorders, movement disorders, and cardiac arrythmia. The electric current is usually delivered by contact electrodes i.e., electrodes that are in Ohmic contact with the biological tissue. An exception is the capacitor electrode of Guyton and Hambrecht 4!, which consists of an implanted porous tantalum disc with a thin insulating coating of tantalum pentoxide. After implantation, the pores fill with extracellular fluid and thus present a large capacitive interface to the fluid. The electrode is capable of delivering sizable currents to tissue without causing accumulation of electrochemical byproducts. Mauro 5! has proposed another capacitor electrode in which one of the "plates" is formed by an electrolyte that is in Ohmic contact with the tissue, via a thin tube. In both these cases the capacitance employed is large, such as to pass currents of a magnitude and duration large enough to cause firing of the nerves, as expressed by the strength-duration curve with typical times of 0.1 ms and currents of the order of 1 mA 6,7!. The nerves fire as a result of substantial depolarization of the nerve membrane by the applied electric current, a process here called classical nerve stimulation.
An area of neurostimulation that has attracted much attention is the induction of relaxation and sleep. One method, called Cranial Electric Stimulation (CES) involves passing an alternating current through the brain via contact electrodes attached to the head or held in the mouth. With properly chosen strength and frequency, these currents may excite or support brain waves that accompany deep sleep. The method has been explored extensively in the Former Soviet Union, under the name "Electrosleep".
A commercially available device is the Japanese "Sleepy" 8!, which generates for one hour square pulses of 4 V and 0.2 ms duration, with a frequency that sweeps from 14 to 0 Hz, every 3 minutes. The device requires contact electrodes placed on the head. Other commercial CES devices 9! are Alpha Stim, Mindman, and Endo Stim, which all require contact electrodes attached to the head.
Electric currents in biological tissue may also be induced by an electric field that is generated in the space outside the subject. The external electric field is set up by applying an electric potential between field electrodes that do not have Ohmic contact with the tissue. Of course the arrangement may be seen as a form of capacitive coupling, but the capacitances are very much smaller than in Mauro 5! or Guyton and Hambrecht 4!. There is also an important practical difference, in that no bodily contact with any part of the apparatus is required for the electric field application by the field electrodes.
A neurological effect of external electric fields has been mentioned by Norbert Wiener 10!, in discussing the bunching of brain waves through nonlinear interactions. The electric field was arranged to provide "a direct electrical driving of the brain" 10!. Wiener describes the field as set up by a 10 Hz alternating voltage of 400 V applied in a room between ceiling and ground.
Brennan 11! describes an apparatus for alleviating disruptions in circadian rythms of a mammal, in which an external alternating electric field is applied across the head of a subject. The voltage applied to the electrodes is specified as at least 100 V, and the peak-to-peak value of the electric field as at least 590 V/m in free air before deploying the electrodes across the head of the subject. The frequency of the alternating electric field is in the range from 5 to 40 Hz. Brennan states that the method is aimed at subjecting at least part of the subject's brain to an alternating electric field, in the belief that this would stimulate an influx of Ca.sup.2+ ions into nerve endings, which in turn would "regulate and facilitate the release of neurotransmitters". Embodiments mentioned include electrodes arranged in a head cap, in a bed, or mounted on the walls of a room. It should be noted that electric polarization of the head causes the field strength in the narrow space between electrode and skin to be about a factor h/2d larger than the free-air field strength, h being; the distance between the electrodes and d the spacing between electrode and skin. For h=17 cm and d=5 mm the factor comes to 17, so that with the specified free-air field of at least 590 V/m the field in the gap between electrode and skin is at least 10 KV/m peak to peak.
A device involving a field electrode as well as a contact electrode is the "Graham Potentializer" mentioned in Ref. 9!. This relaxation device uses motion, light and sound as well as an external alternating electric field applied predominantly to the head. The contact electrode is a metal bar in Ohmic contact with the bare feet of the subject; the field electrode is a hemispherical metal headpiece placed several inches from the subject's head. According to the brief description in 9!, a signal less than 2 V at a frequency of 125 Hz is applied between the field electrode and the contact electrode. In this configuration the contact electrode supplies to the body the current for charging the capacitor formed by the field electrode and the apposing skin area. The resulting electric field stands predominantly in the space between the head piece and the scalp.
In the three external field methods mentioned, viz. Wiener 10!, Brennan 11!, and Graham 9!, the electric field is applied to the head, thereby subjecting the brain to polarization currents. These currents run through the brain in a broad swath, with a distribution influenced by nonuniformities of conductivity and permittivity. The scale of the current density can be conveniently expressed by the maximum value, over the skin of the head, of its component perpendicular to the local skin. This scale is easily calculated for sinusoidal fields as the product of radian frequency, permittivity, and maximum amplitude of the external field on the head. Using Brennan's 11! lowest frequency of 5 Hz, his minimum required free-air field strength of 590 V/m, and the factor 17 as estimated above to account for the polarization of the head by the applied field, the scale of the polarization current density in the brain comes to about 280 pA/cm.sup.2. Without understanding the neurological effects involved, it is prudent to avoid exposing the brain to current densities of such scale, and impose as a limit 1/4000 times the scale calculated for Brennan's patent. Polarization current densities in the brain with a scale in excess of 70 fA/cm.sup.2 are henceforth considered substantial.
It is the object of the present invention to obtain a method and apparatus for manipulating the nervous system by external electric fields without causing substantial polarization currents in the brain.
The use of electric fields raises concerns about possible health effects. Such concerns have been widely discussed in the media in regard to electric power lines and electric apparatus 12!. Answering the pertinent questions by objective research will take time, but meanwhile governments have been setting guidelines for safe limits on field strengths. At present, the strictest limits of this sort are the Swedish MPRII guidelines. Magnetic fields are of no concern here, because the currents involved are so small. However, the electric field strengthy must be considered, since even at low voltages strong electric fields can result from electrodes placed close to the skin. For extremely low frequency fields, the MPRII guidelines limit the field strength to 25 V/m in the frequency range from 5 Hz to 2 KHz. In the Brennan patent 11! the minimum field strength of 590 V/m violates the guidelines by a factor 23; when the polarization effects are accounted for, the factor is about 400.
It is a further object of the present invention to manipulate the nervous system by external electric fields that are in compliance with the MPRII guidelines.
Brennan 11! stipulates voltages of at least 100 V, and as high as 600 V for his preferred embodiment. Generation of such voltages requires a voltage multiplier stage, if practical battery operation is desired. This increases the current drain and the size of the generator. The large voltages also raise safety concerns.
It is yet a further object of the present invention to manipulate the nervous system by external electric fields, using low voltages that are generated by a small and safe battery-powered device with low current consumption.